I attended the American Carbon Society Symposium and
Workshop on thermal management, held at
North Carolina State University, March
18 and 19, 2024. I was an invited speaker at that
conference, and I took 3
presentations, ready to present. Only one was “live”, we turned the other two into poster
presentations on-site.
The live presentation (photo) had to do with old-style
by-hand methods of design analysis being used up-front for concept
screening, to enable efficient use of
the concept brainstorming process to increase chances of project success. That enables concentrating the real design
efforts and heavy-duty design analyses with software packages, to be reserved for only the one or two best
concepts, thus using resources and
schedule time efficiently. Analysts who
can do this sort of by-hand analysis can also more readily-recognize “garbage-in, garbage-out” problems with software packages!
The specific example used for this presentation was the old
H. Julian Allen by-hand simplified re-entry analysis, used for warhead design about 1953, and declassified in the late 1950’s. I have taken that analysis and re-implemented
it in the form of an Excel spreadsheet file,
with worksheets representing the atmospheres of Earth, Mars,
and Titan. Those models came from
the Justus and Braun paper regarding entry,
descent, and landing, presented several years ago.
There are only 4 easily-estimated pieces of data required to
represent the entering object: its speed
at entry interface, its trajectory angle
below horizontal at entry interface, its
hypersonic ballistic coefficient, and
its effective nose radius which determines how bad the stagnation heating will
be. The altitude at entry interface is
part of the atmosphere model. My
spreadsheet creates plots, the most
useful of which tell you the peak heating rate,
followed closely in time by the peak deceleration gees.
Those lead easily and immediately to the peak pressure on
the heat shield material, and (by way of
a thermal balance) the surface temperature that must be withstood. Those in turn constrain your material
selection.
The other two presentations were about a unique ceramic
composite heat shield material I created out of essentially hardware-store
materials, decades ago, and about the ramjet combustor ablative
materials I tested decades ago, in a
particularly-productive direct-connect test series.
The spreadsheet entry analysis, and others about orbital mechanics, compressible flow, high speed heat transfer, and rocket engine performance, are all things I can make available. Contact me.
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Up to this point, I
was able to post the same remarks on LinkedIn and stay within a 400 word
limit. Here on “exrocketman” I can say
more and provide more informative detail.
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The entry spreadsheet uses worksheets with the atmosphere
models already set up for each of three worlds:
Earth, Mars, and Titan.
All use the same stagnation heating model. There are only 4 inputs needed to model an
entering vehicle. It generates plots automatically, but you need to make sure the altitude data
in the worksheet do not go past something very close to the Mach 3 point. Or else you would have to recreate the plots
from scratch, limiting what data you
select for plotting to the Mach 3 point,
in order to prevent extreme scale distortion. This is what the Mars entry worksheet looks
like:
This old model is 2-D Cartesian (you have to “wrap” its
results around the planet). The
trajectory has a constant angle Ɵ
with respect to horizontal, making it a
simple straight line (in the real world,
it will “droop” significantly after the peak deceleration pulse). It uses a very simple scale-height type of
exponential model for density variation with altitude: ρ = ρ0 exp[h/hscale], where ρ0 and hscale are
merely the curve fit constants for modeling density in the altitude range of
interest. It presumes a constant
hypersonic ballistic coefficient β = Mentry/(CD Ablock), which for blunt shapes means the entry
analysis math assumptions are violated below local Mach 3. Allen came up with a simple closed-form double-exponential
equation modeling speed versus altitude,
under these particular assumptions:
V = Vatm exp{-C exp[h/hscale]},
where Vatm is the object’s speed at entry
interface, C = 1000*ρ0*hscale/(2*β*sin
Ɵ), and the analysis starts downward from h = hatm, the altitude for entry interface (a property
of the atmosphere model along with ρ0 and hscale). The factor of 1000 converts the customary km
units of hscale to m. While
the equations create results at speeds under local Mach 3, they are in error for not being hypersonic (β
is no longer constant), and those points
should not be included in any reported results or plots.
Allen used a stagnation convective heating correlation that
is surprisingly accurate, even
today. It is q = Q/A = 1.75 x 10-8
(ρ/RN)0.5 (1000*V, km/s)3, where ρ is measured in kg/m3, and RN is the effective nose
radius in meters. The value of Q/A =
q is measured in Watts/cm2. Its integral with time is in the
spreadsheet. This is convective heating
only, one would have to add a model for
plasma sheath radiation heating, for
speeds at entry exceeding about 9 km/s.
That is currently not in the spreadsheet, but is considered to be negligible at entry
interface speeds of 8 km/s and less. The
analysis is summarized in this figure:
Where Do-It-Yourselfers Can Obtain Such Materials
At least the entry spreadsheet, the orbital mechanics spreadsheet, and one version of the rocket engine
performance spreadsheet, can downloaded
for free, using links that are on the
Mars Society’s “New Mars” forums site: newmars.com/forums/
These are located on that forums site in the “Acheron Labs”
section, under the topic “Interplanetary
transportation”. Scroll down a page or
two, to the thread titled “orbital
mechanics class traditional”. The list
of available lessons is in the first posting there. Subsequent posts have the links to all
the lessons, which are actually located in
a drop box on-line. All three named-above
spreadsheets are available from that drop box,
as part of the supplied class materials for this course.
The course comprises multiple lessons that acquaint the
student with classical 2-body orbital mechanics of elliptic orbits, to include interplanetary transfers, adds in empirical corrections for losses
during launch and when 3 bodies are involved,
acquaints you with entry,
descent, and landing issues, then takes up rocket vehicle performance
estimation (and the rocket engine performance estimation methods to support
it).
Be aware that I have two other courses not available
from this New Mars forums site, but
instead directly from me. One is
about compressible flow, to include flow
with losses and with heat addition, as
well as shock waves and expansion fans,
plus the same rocket engine performance estimations as are in one of the
orbits course lessons. The other has to
do with high-speed heat transfer,
complete with recommended models for various situations. Both of these courses are associated with
spreadsheets as part of the class materials.
All these class materials include pdf documents that are
essentially texts from which to teach yourself how to do these things. They include demonstration problems with
solutions, and assigned problems to be
worked, plus solutions to those assigned
problems, for comparison
afterwards. For the already-adept, there are also slide shows from which you can
teach others.
GW’s Ramjet Book
Also be aware that I have offered my ramjet book “A
Practical Guide to Ramjet Propulsion” as a self-published item. Just contact me by email, it currently comes as a series of pdf
files, which I email to you upon receipt
of payment. I hope to soon have a fully
automated site, with a final single
download file for the book. This
is not an academic work, it is a
real “how-to” guide written from my direct experiences doing that kind of work
in the aerospace/defense industry long ago. It deals with plain subsonic-combustion
ramjets, to include integral
boosters, but not ejector ramjets, combined cycles, or supersonic combustion. If that interests you, please contact me (email preferred).
Other Technical Articles Posted On “Exrocketman”
There are many technical articles on a variety of topics
posted here on “exrocketman”, along with
a few things posted on youtube under the channel name “exrocketman1”. Here on “exrocketman” the blog site, there is a catalog article posted, that I try to keep current, which has these things as a list for each one
of multiple topic areas. This is the
article “Lists of Some Articles by Topic Area”,
posted 21 October 2021.
All you need are the posting date and title of the article
you seek, to find anything quickly on
this site, using the blog archive tool, left side of page. Click on the year, then the month, then the title if need be (such as if
multiple articles were posted that month).
Just peruse the lists and jot down the dates and titles you want to
see, then use the archive tool.
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Your calculations seem to suggest just a standard cylindrical rocket stage, such as the SpaceX Starship, should be able to slow down enough to need only a small amount of propulsive slowing on Mars reentry, perhaps ca. 700 m/s needs to be cancelled out by propulsion.
ReplyDeletePuzzling then why NASA takes it to be such a difficult problem to solve:
So perhaps about 0.7km/s that needs to be cancelled out by rockets for a 150 ton lander. Even if the higher estimate of 1.1 km/s is needed, that’s not particularly difficult. I wonder why NASA considered landing large, crewed habitats on Mars such a daunting problem:
The Mars Landing Approach: Getting Large Payloads to the Surface of the Red Planet.
POSTED ON JULY 17, 2007 BY NANCY ATKINSON
"Some proponents of human missions to Mars say we have the technology today to send people to the Red Planet. But do we? Rob Manning of the Jet Propulsion Laboratory discusses the intricacies of entry, descent and landing and what needs to be done to make humans on Mars a reality."
"There’s no comfort in the statistics for missions to Mars. To date over 60% of the missions have failed. The scientists and engineers of these undertakings use phrases like “Six Minutes of Terror,” and “The Great Galactic Ghoul” to illustrate their experiences, evidence of the anxiety that’s evoked by sending a robotic spacecraft to Mars — even among those who have devoted their careers to the task. But mention sending a human mission to land on the Red Planet, with payloads several factors larger than an unmanned spacecraft and the trepidation among that same group grows even larger. Why?"
"Nobody knows how to do it."
https://www.universetoday.com/7024/the-mars-landing-approach-getting-large-payloads-to-the-surface-of-the-red-planet/
Bob Clark
NASA has never thought outside the box they have used for decades with small probes. That's high altitude at end of hypersonics, followed by a chute, with only terminal rocket braking because of the high near-sonic terminal chute speed. But at high ballistic coefficient (inherent with bigger masses !!!), you come out of hypersonics low, not high, and cannot use a chute at all, because there is not time to deploy it, much less get any deceleration from it. Landing with no chute is something NASA has never before done on Mars. They are scared to death to make the change. -- GW
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